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zram: fix the issue that the write - back limits might overflow

When the page size exceeds 4KB, if bd_wb_limit is set to a value that is
not aligned with the page size, it will cause a numerical wrap-around
issue for bd_wb_limit. For example, when the page size is set to 16KB and
bd_wb_limit is set to 3, after one write-back operation, the value of
bd_wb_limit will become -1. More seriously, since bd_wb_limit is an
unsigned number, its value may become as large as 2^64 - 1.

The core reason for this problem is that the unit of bd_wb_limit is 4KB.
For example, when a write-back occurs on a system with a page size of
16KB, 4 needs to be subtracted from bd_wb_limit. This operation takes
place in the zram_account_writeback_submit function.

This patch fixes the issue by limiting bd_wb_limit to be an integer
multiple of PAGE_SIZE / 4096.

Link: https://lkml.kernel.org/r/tencent_5936CFE72BAB2BA76887BB69DCC1B5E67C05@qq.com
Fixes: 1d69a3f8ae77 ("zram: idle writeback fixes and cleanup")
Signed-off-by: Yuwen Chen <ywen.chen@foxmail.com>
Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Richard Chang <richardycc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

authored by

Yuwen Chen and committed by
Andrew Morton 04d31610 ecf371b2

+10
+10
drivers/block/zram/zram_drv.c
··· 566 566 if (kstrtoull(buf, 10, &val)) 567 567 return ret; 568 568 569 + /* 570 + * When the page size is greater than 4KB, if bd_wb_limit is set to 571 + * a value that is not page - size aligned, it will cause value 572 + * wrapping. For example, when the page size is set to 16KB and 573 + * bd_wb_limit is set to 3, a single write - back operation will 574 + * cause bd_wb_limit to become -1. Even more terrifying is that 575 + * bd_wb_limit is an unsigned number. 576 + */ 577 + val = rounddown(val, PAGE_SIZE / 4096); 578 + 569 579 down_write(&zram->init_lock); 570 580 zram->bd_wb_limit = val; 571 581 up_write(&zram->init_lock);